Reapply commit r258404 with fix

This patch is the second attempt to reapply commit r258404. There was bug in
the initial patch and subsequent fix (mentioned below).

The initial patch caused an assertion because we were computing smaller type
sizes for instructions that cannot be demoted. The fix first determines the
instructions that will be demoted, and then applies the smaller type size to
only those instructions.

This should fix PR26239 and PR26307.

llvm-svn: 258929